Concentration-dependent nitrogen functionalization of graphene nanoplatelets: structural, surface, and electrical properties

Nitrogen-functionalized graphene nanoplatelets (N-GNPs) were synthesized by a low-temperature dopamine/HCl-assisted microwave method using 1-5% nominal N precursor concentrations to investigate concentration-dependent structural evolution. The materials were characterized by SEM, TEM, EDX, XRD, FTIR, Raman, XPS, BET analysis, and electrical measurements. SEM revealed thin, wrinkled multilayer graphene sheets suitable for N functionalization. XRD confirmed preservation of the graphitic framework, while the interlayer spacing changed non-monotonically from 0.33724 nm (pure) to 0.33924 nm (1%) and 0.33576 nm (5%), reflecting competition between N-induced expansion and π-π restacking. FTIR confirmed the formation of C-N/C[double bond, length as m-dash]N and N-H functional groups. Raman analysis showed maximum defect generation for the 2% sample, where the I _D/I _G ratio increased from 0.243 to 0.492, and the apparent graphitic domain size decreased from 79.1 to 39.1 nm, while I _2D/I _G < 1 confirmed preservation of the multilayer structure. XPS N 1s spectra revealed two components at binding energy ∼399.6 and ∼401.5 eV, attributed to pyrrolic/amine-type and graphitic N or/and/or protonated amine species, respectively, supporting N modification of the GNP surface. BET analysis showed the highest specific surface area (45.6 m² g^-1) for the GNP-N 2% sample. Electrical measurements revealed a non-linear dependence on N content, with N-GNPs 2% showing the highest conductivity among the functionalized samples (72.73 S cm^-1), although remaining below pure GNPs (88.50 S cm^-1). Combined with its maximum BET surface area, this composition (2%) provides the most favorable balance among the measured surface, structural, and electrical properties within the N-functionalized series.